Pilot pattern design for a STTD scheme in an OFDM system

ABSTRACT

A transmitting device for transmitting data symbols and pilot symbols in an OFDM transmission system; the device comprising symbol generating means for generating said data symbols and said pilot symbols, means for transmitting said data symbols and pilot symbols respectively by using a plurality of subcarriers of said OFDM transmission system, wherein said symbol generating means is designed to selectively generate a first type pilot symbol and a second type pilot symbol being orthogonal to said first type pilot symbol so that a pilot symbol pattern in the frequency dimension comprises at least said first type pilot symbol to be transmitted by using a predefined subcarrier and second type pilot symbol to be transmitted by using other predefined subcarrier, and wherein said pilot symbol pattern has a different pattern from a succeeding pilot symbol pattern in time dimension.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and is based upon and claims thebenefit of priority under 35 U.S.C. § 120 from U.S. Ser. No. 15/420,150,filed Jan. 31, 2017, which is a continuation of U.S. Ser. No.15/171,176, filed Jun. 2, 2016 (now U.S. Pat. No. 9,596,113), which is acontinuation of U.S. Ser. No. 14/591,433, filed Jan. 7, 2015 (now U.S.Pat. No. 9,369,323), which is a continuation of Ser. No. 13/850,017,filed Mar. 25, 2013 Ser. No. 12/769,958, filed Apr. 29, 2010 (now U.S.Pat. No. 8,532,133) which is a continuation of U.S. Ser. No. 12/250,306,filed Oct. 13, 2008 (now U.S. Pat. No. 7,746,759) which is a divisionalof U.S. Ser. No. 11/338,644, filed Jan. 25, 2006 (now U.S. Pat. No.7,646,700) which is a divisional of U.S. Ser. No. 09/898,389, filed Jul.3, 2001 (now U.S. Pat. No. 7,221,645) and claims the benefit of priorityfrom European Patent Application No. 00 114 423.7, filed Jul. 5, 2000;the entire contents of each of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a transmitting device and a receivingdevice of a wireless orthogonal frequency division multiplex (OFDM)communication system.

Description of the Related Art

In wireless telecommunication, the transmission quality between atransmitting device, such as a base station, and a receiving device,such as a mobile terminal, depends strongly on the respectivetransmission environment and is often deteriorated by fading effects andthe like. This often leads to poor speech and data transmission quality,particularly if only one single antenna is used on the transmission sideand one single antenna is used on the receiving side. Therefore, somemobile terminals for wireless telecommunication systems, such as the GSMsystem, comprise two or more and different kinds of antennas built asinternal or external antennas in the mobile terminal. However, it isdesirable that modern mobile terminals are as small and light aspossible and therefore it is an increasing interest to use only a singleantenna in these mobile terminals. In order to allow the use of only asingle antenna on the receiving side, particularly the mobile terminalside, it has been proposed to use more than one antenna on thetransmitting side, particularly the base station side, so that thediversity gain can be used for a better transmission quality. Thisscheme is called transmit diversity. Transmit diversity generally meansthat more than one antenna, e.g. two antennas, transmit datasimultaneously to a receiving device. If the same data are transmittedin parallel by two antennas, the receiving side has a chance to receivesignals at least from one of the antennas with an acceptabletransmission quality so that a good connection can be ensured. Onespecific approach in the transmit diversity scheme is the use of aso-called space time coding. The resulting space time transmit diversity(STTD) has been adapted and is of the UMTS standard for the nextgeneration of mobile telecommunication part

In a space time transmit diversity system, a transmitting device, suchas a base station, comprises e.g. two antennas arranged spaced apartfrom each other in a space diversity arrangement. A stream of data to betransmitted to a receiving device, such as a mobile terminal, is encodedand processed so that two parallel data streams are generated. Afterfurther processing corresponding to the respective wirelesscommunication system, the data of each of the two data streams aretransmitted by a respective one of the two antennas. Although generallythe same data content is transmitted by each of the two antennas, thesignals transmitted by the two antennas are not absolutely identical,but data symbols to be transmitted are mapped or coded slightlydifferently on the signals transmitted by each of the antennas. Thisallows a receiving device receiving the signals transmitted from the twoantennas with only a single antenna to distinguish and separate signalscoming from one of the transmitting antennas from signals coming fromthe other of the transmitting antennas. Since the two transmittingantennas are arranged in a space diversity arrangement, crossinterference is avoided and the receiving device can then, after acorresponding channel estimation, distinguish and combine the signalsfrom the two transmitting antennas to obtain a better transmissionquality. The channel estimation in the receiving device is usuallyperformed on the basis of pilot symbols transmitted from thetransmitting device. The receiving device performs a channel estimationby comparing received pilot symbols with an expected pilot symbol tomeasure the channel response and to tune the receiving device to thebest transmission channel, i.e. the transmitting antenna to which thebetter connection exists.

The above-mentioned UMTS system bases on a code division multiple access(CDMA) scheme. The CDMA scheme is only one of several possible multipleaccess schemes used in wireless telecommunication. For wirelesstelecommunication with high data rates, the orthogonal frequencydivision multiplex (OFDM) scheme is known, in which the availablefrequency band used for a communication is divided in a plurality offrequency subcarriers, whereby adjacent frequency subcarriers arerespectively orthogonal to each other.

SUMMARY OF THE INVENTION

The object of the present invention is now to propose a transmittingdevice, which allows a simple and effective channel estimation to beperformed.

This object is achieved by a transmitting device according to theindependent claims.

The present invention relates to a transmitting device for transmittingdata symbols and pilot symbols in an OFDM transmission system; thedevice comprising symbol generating means for generating said datasymbols and said pilot symbols,

means for transmitting said data symbols and pilot symbols respectivelyby using a plurality of subcarriers of said OFDM transmission system,

wherein said symbol generating means is designed to selectively generatea first type pilot symbol and a second type pilot symbol beingorthogonal to said first type pilot symbol so that a pilot symbolpattern in the frequency dimension comprises at least said first typepilot symbol to be transmitted by using a predefined subcarrier andsecond type pilot symbol to be transmitted by using other predefinedsubcarrier, and

wherein said pilot symbol pattern has a different pattern from asucceeding pilot symbol pattern in time dimension.

The present invention further relates to a transmitting device fortransmitting data symbols and pilot symbols in an OFDM transmissionsystem; the device comprising

symbol generating means for generating said data symbols and said pilotsymbols,

means for transmitting said data symbols and pilot symbols by using aplurality of subcarriers of said OFDM transmission system,

wherein said symbol generating means is designed to selectively generatea first type pilot symbol and a second type pilot symbol beingorthogonal to said first type pilot symbol so as to create a pilotsymbol pattern in which said first and second type pilot symbols areallocated respectively in said frequency dimension, and

wherein said pilot symbol pattern has a different pattern from asucceeding pilot symbol pattern in time dimension.

The present invention further relates to a transmitting device fortransmitting data symbols and pilot symbols in an OFDM transmissionsystem; the device comprising

symbol generating means for generating said data symbols and said pilotsymbols,

means for transmitting said data symbols and pilot symbols by using aplurality of subcarriers of said OFDM transmission system,

wherein said symbol generating means is designed to regularly allocateeither a first type pilot symbol or a second type pilot symbol beingorthogonal to said first type pilot symbol in the frequency dimension tosaid generated pilot symbols.

The present invention further relates to a transmitting device fortransmitting data symbols and pilot symbols in an OFDM transmissionsystem; the device comprising

symbol generating means for generating said data symbols and said pilotsymbols,

means for transmitting said data symbols and pilot symbols respectivelyby using a plurality of subcarriers of said OFDM transmission system,

wherein said symbol generating means is designed to selectively generatea first type pilot symbol and a second type pilot symbol beingorthogonal to said first type pilot symbol so that a pilot symbolpattern in the time dimension comprises at least said first and secondtype pilot symbols to be transmitted at different timepointsrespectively,

wherein said pilot symbol pattern to be transmitted by using one of saidplurality of subcarriers is different from a pilot symbol pattern to betransmitted by using an adjacent subcarrier.

The present invention further relates to a transmitting device fortransmitting data symbols and pilot symbols in an OFDM transmissionsystem; the device comprising

symbol generating means for generating said data symbols and said pilotsymbols,

means for transmitting said data symbols and pilot symbols by using aplurality of subcarriers of said OFDM transmission system,

wherein said symbol generating means is designed to selectively generatea first type pilot symbol and a second type pilot symbol beingorthogonal to said first type pilot symbol so as to create a pilotsymbol pattern in which said first and second type pilot symbols areallocated regularly in the time dimension, and

wherein said pilot symbol pattern to be transmitted by using one of saidplurality of subcarriers is different from a pilot symbol pattern to betransmitted by using an adjacent subcarrier.

The present invention further relates to a transmitting device fortransmitting data symbols and pilot symbols in an OFDM transmissionsystem; the device comprising

symbol generating means for generating said data symbols and said pilotsymbols,

means for transmitting said data symbols and pilot symbols by using aplurality of subcarriers of said OFDM transmission system,

wherein a pilot symbol pattern, in which a first type pilot symbol and asecond type pilot symbol being orthogonal to said first type pilotsymbol are allocated in the time dimension, to be transmitted by usingone of said plurality of subcarriers is different from a pilot symbolpattern to be transmitted by using an adjacent subcarrier.

Advantageously, corresponding first and second pilot symbols have thesame frequency and time allocation in the OFDM system. In other words,corresponding first and second pilot symbols are transmitted in the samesubcarrier and the same timeslot of the OFDM system. Hereby, furtheradvantageously corresponding first and second pilot symbols having thesame frequency and time allocation are alternatingly identical andorthogonal to each other in the frequency as well as in the timedimension. This means that in the frequency and time grid of the OFDMsystem, identical first and second pilot symbols and orthogonal firstand second pilot symbols alternate with each other in the frequency aswell as the time dimension.

Advantageously, pairs of first pilot symbols being adjacent in the timedimension are respectively orthogonal to the corresponding pairs ofsecond pilot symbols.

Advantageously, pairs of first pilot symbols being adjacent in thefrequency dimension are respectively orthogonal to the correspondingpairs of second pilot symbols.

Advantageously, the first and the second pilot symbols have a regulardistribution in the time and the frequency dimension, whereby the secondpilot symbols alternately have the identical and the inverse complexvalue of the corresponding first pilot symbol in the time as well as inthe frequency dimension.

The proposed scheme of transmitting, receiving and processing first andsecond pilot symbols allows a simple and effective channel estimationprocessing to be performed on the receiving side so that a bettercoherent demodulation of the transmission channel can be performed toensure the best transmission quality. In an advantageous aspect, thepresent invention ensures full space and time diversity. Further, nofeedback information from the receiving side to the transmitting side isrequired and an improved data transmission capacity can be realised.Further, the proposed system is robust to transmission antenna failuresand guarantees power amplifier balance on the transmitting side.

It has to be clarified at this point that the single antenna of areceiving device receives the first pilot symbols transmitted from thefirst antenna means and the second pilot symbols transmitted from thesecond antenna means of the transmitting device only as a combined orsuperimposed pilot symbol. In case that the first pilot symbol and thesecond pilot symbol transmitted in the same frequency subcarrier and thesame timepoint are identical, the receiving device receives a combinedpilot symbol comprising the superimposed identical first and secondpilot symbol. In case that the first and second pilot symbol areorthogonal to each other, the receiving device receives a combined pilotsymbol comprising the superimposed orthogonal first and second pilotsymbol. In the receiving device, the transfer function of the first andthe second pilot symbol, respectively, can therefore be separated sothat the respective channel estimation for each of the two transmissionantennas can be performed in a simple way.

Advantageously, the second pilot symbols alternatingly have theidentical and the inverse complex value of the corresponding first pilotsymbol in the time as well as in the frequency dimension, so that theprocessing and the channel estimation on the receiving side can beperformed on a basis of a simple addition and subtraction calculation ofthe received pilot symbols. On the basis of the channel estimationresult, both signals from the first antenna means and from the secondantenna means of the transmitting device are further processed and usedas the communication data in the receiving device.

The transmitting device according to the present invention can e.g. beimplemented in the base station of an OFDM communication system or in amobile terminal of an OFDM communication system.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following description, the present invention is explained in moredetail in relation to the enclosed drawings, in which

FIG. 1 shows schematically a base station comprising a transmittingdevice according to the present invention,

FIG. 2 shows schematically a mobile terminal comprising a receivingdevice according to the present invention,

FIGS. 3A and 3B a first and a second example, respectively, of a pilotsymbol pattern transmitted by a first antenna means of a transmittingdevice according to the present invention, and

FIGS. 4A and 4B a first and a second example, respectively, of a pilotsymbol pattern transmitted by the second antenna means of thetransmitting device according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the block diagram of a base station 1 of a wirelessorthogonal frequency division multiplex (OFDM) communication system isshown, which comprises a transmitting device according to the presentinvention. It is to be understood that in FIG. 1 only elements importantfor the understanding of the present invention are shown. Furtherelements, such as coding means, modulation means, RF part and the likenecessary for the operation of the base station are omitted for the sakeof clarity.

The base station 1 comprises a first antenna 5 and a second antenna 6being arranged spaced apart from each other, e.g. in a space diversityarrangement. In this case, the first antenna 5 may also be called anon-diversity antenna and the second antenna 6 can also be called adiversity antenna. The space diversity arrangement of the first antenna5 and the second antenna 6 is so that the two antennas 5 and 6 aresufficiently separated in space, so that the signals transmitted by thefirst antenna 5 and the second antenna 6, respectively, are uncorrelatedand an effective diversity gain can be achieved on the receiving side.

Further, the base station 1 comprises a encoding means 3 for encoding adata stream, e.g. on the basis of a space time transmit diversity (STTD)scheme and outputting a first and a second STTD encoded data stream to amultiplexer 4. The first STTD encoded data stream is to be transmittedvia the first antenna 5 and the second STTD encoded data stream is to betransmitted via the second antenna 6. Although the data transmitted fromthe first antenna 5 and the second antenna 6 are generally the samedata, i.e. contain the data of the single data stream supplied to theencoding means 3, the data are not transmitted identically by the twoantennas 5 and 6. For example, the data transmitted by the first antenna5 identically correspond to the data arrangement of the single datastream supplied to the encoding means 3. If, e.g. a first data symbol S₁in a time period 0-T and a second data symbol S₂ in the succeeding timeperiod T−2T are supplied to the encoding means 3, the first data streamoutput by the encoding means can identically correspond to thatarrangement (data symbol S₁ followed by data symbol S₂). The second datastream output by the encoding means 3, however, contains the datasymbols S₁ and S₂ in a different arrangement. For example, as shown inFIG. 1, in the second data stream, the data symbol of the first timeperiod 0-T could be the negative complex conjugated value of the seconddata block S₂ of the first data stream, i.e. −S*₂. The next succeedingdata symbol of the second data stream is the conjugated complex value ofthe first data symbol S₁ of the first data stream, i.e. S*₁. Thus, thesecond data stream contains the identical data content as the first datastream, but in a different arrangement. A receiving device receiving thesignals from the first antenna 5 and the second antenna 6 assuperimposed signals is therefore able to clearly distinguish betweenthe signals transmitted from the first antenna 5 and the signalstransmitted from the second antenna 6 due to the space diversityarrangement and the different arrangement of the same data content. Itis to be understood that the space time transmit diversity scheme shownin and explained in relation to FIG. 1 only serves as an example toexplain the present invention. Any other STTD scheme for transmittingdata via the first antenna 5 and the second antenna 6 can be applied.

The base station 1 further comprises a pilot symbol generating means 2for generating pilot symbols to be transmitted among the data of thefirst and the second data stream by the first antenna 5 and the secondantenna 6. Thereby, the pilot symbol generating means 2 generates andsupplies different pilot symbol patterns to be transmitted via the firstantenna 5 and the second antenna 6, respectively, to the multiplexer 4.The general idea of the present invention is that some of the pilotsymbols transmitted by the first antenna 5 and the second antenna 6 areorthogonal to each other so that the cross-interference from bothantennas 5 and 6 is eliminated, the signals from the first,(non-diversity) antenna 5 and the second (diversity) antenna 6 can bedifferentiated and consequently a separate channel estimation for eachantenna 5, 6 can be achieved in a receiving device.

FIG. 2 shows a schematic block diagram of a mobile terminal 10comprising a receiving device for receiving signals in a wireless OFDMcommunication system according to the present invention. Particularly,the mobile terminal 10 is adapted to receive signals from a base station1 as shown in FIG. 1.

The mobile terminal 10 comprises a single antenna 11 for receiving STTDencoded signals as well as pilot symbols transmitted from the firstantenna 5 and the second antenna 6 of the base station 1. Further, themobile terminal 10 comprises a receiving means 12, which comprises e.g.the necessary RF part and the like. Further, the mobile terminal 10comprises a demodulation means for demodulating signals received by thereceiving means 12 via the antenna 11. It is to be understood that themobile terminal 10 further comprises all necessary elements to beoperated in the corresponding wireless OFDM system. However, theseelements are not shown for the sake of clarity.

The mobile terminal 10 further comprises a processing means 14 fordetecting pilot symbols in the signals received by the receiving means12 via the antenna 11. The processing means 14 processes detected pilotsymbols and performs a channel estimation on the basis of the processingto separately determine the transmission quality of the received signalstransmitted from the first antenna 5 and the second antenna 6,respectively. In other words, by processing the received pilot symbols,which are combined pilot symbols comprising the first and the secondpilot symbols simultaneously transmitted by the first antenna 5 and thesecond antenna 6, the processing means 14 is able to separatelydetermine the transmission quality of the signals transmitted from thefirst antenna 5 and the transmission quality of the signals transmittedfrom the second antenna 6. On the basis of this channel estimationresult, both the STTD encoded signals from the first antenna 5 and fromthe second antenna 6 are further processed and used as communicationdata in the mobile terminal 10.

As stated above, at least some of the second pilot symbols transmittedfrom the second antenna 6 are orthogonal to corresponding first pilotsymbols transmitted by the antenna 5. The processing performed in theprocessing means 14 bases on this orthogonality of the first and thesecond pilot symbols and enables the separate channel estimation for thefirst and the second antenna 5 and 6, respectively. In relation to FIGS.3 and 4, a specific example for pilot symbol patterns to be transmittedby the base station 1 and to be received and processed in the mobileterminal 10 are proposed.

FIG. 3 comprises two FIGS. 3A and 3B. FIG. 3A shows a first example of apilot symbol pattern to be transmitted by the first (non-diversity)antenna 5 of the base station 1. The shown pilot symbol pattern has aregular distribution in the time and the frequency dimension of the OFDMsystem. The pilot symbols 20, 21, . . . , 28 are always transmitted inthe same frequency subcarriers and in equidistant timepoints. Forexample, the pilot symbols 20, 21 and 22 are transmitted in a firstfrequency subcarrier, whereby respectively four data symbols aretransmitted between adjacent pilot symbols 20, 21 and 21, 22. Pilotpatterns 23, 24 and 25 are transmitted in a second frequency subcarrierand the pilot symbol 26, 27 and 28 are transmitted in a third frequencysubcarrier. Thereby, the pilot symbols 20, 23 and 26 are transmitted atthe same first timepoint, the pilot symbols 21, 24 and 27 aretransmitted in the same second timepoint and the pilot symbols 22, 25and 28 are transmitted in the same third timepoint. Thus, always thesame frequency subcarriers are used for the transmission of the pilotsymbols and the transmission of the pilot symbols in the respectivesubcarriers always takes place at equidistant timepoints. Such a pilotsymbol pattern is known from prior art OFDM systems. On the receivingside, the channel estimation for the data symbols between adjacent pilotsymbols (in frequency and time) is performed by e.g. linearinterpolation. For example, for the data symbols between the pilotsymbols 20 and 21 in the same frequency subcarrier, a linearinterpolation of the pilot symbols 20 and 21 is performed on thereceiving side. For the data symbols between the adjacent pilot symbols20 and 23 received at the same timepoint but in different frequencysubcarriers, a linear interpolation is also performed. For data symbolsin frequency subcarriers, in which no pilot symbols are transmitted, acombination of a time and a frequency interpolation of the respectiveadjacent pilot symbols is performed.

FIG. 3B shows also a regular distribution of the first pilot symbols tobe transmitted by the first antenna 5 of the base station 1. Thedifference to the pilot symbol pattern of FIG. 3A is here that the (intime) succeeding pilot symbols are not transmitted in the same frequencysubcarrier as the preceding pilot symbol, but in the immediatelyadjacent subcarrier. For example, the pilot symbol 31 is not transmittedin the same frequency subcarrier as the preceding pilot symbol 30, butthe immediately adjacent (lower) frequency subcarrier. This pilot symbolpattern may allow a more accurate channel estimation for data symbols offrequency subcarriers, in which no pilot symbols are transmitted.Identical to the pilot symbol pattern proposed in FIG. 3A, the pilotsymbols of the pilot symbol pattern proposed in FIG. 3B are alsotransmitted at identical timepoints. Thus, pilot symbols 30, 34 and 38are transmitted at the first identical timepoint, pilot symbols 31, 35and 39 are transmitted at the same second timepoint, pilot symbols 32,26 and 40 are transmitted at the same third timepoint and pilot symbols33, 37 and 41 are transmitted at the same fourth timepoint.

FIG. 4 comprises two FIGS. 4A and 4B, whereby FIG. 4A shows the pilotsymbol pattern for the second pilot symbols to be transmitted by thesecond antenna 6 of the base station 1, which corresponds to the pilotsymbol pattern of the first pilot symbols shown in FIG. 3A. As can beseen, also the pilot symbol pattern of FIG. 4A shows a very regulardistribution of pilot symbols 42, 43, . . . , 53 in frequency and time.The second pilot symbols are always transmitted in the same frequencysubcarrier and at the same timepoint as the corresponding first pilotsymbol. For example, the second pilot symbol 42 is transmitted in thesame frequency subcarrier and at the same timepoint as the correspondingfirst pilot symbol 20. The second pilot symbol 43 is transmitted in thesame frequency subcarrier and at the same timepoint as the first pilotsymbol 21. The second pilot symbol 46 corresponds to the first pilotsymbol 23, the second pilot symbol 50 corresponds to the first pilotsymbol 26 and so on. Thereby, the second pilot symbols of the pilotsymbol pattern in FIG. 4A are alternatingly identical and orthogonal tothe corresponding first pilot symbols of the pilot symbol pattern shownin FIG. 3A. The second pilot symbols 42, 44, 47, 50 and 52 are identicalto their corresponding first pilot symbols 20, 22, 24, 26 and 28.However, every other second pilot symbol (in time and frequencydimension) is the inverse complex value of the corresponding first pilotsymbol. For example, a second pilot symbol 43 is the inverse complexvalue of the first pilot symbol 21, the second pilot symbol 46 is theinverse complex value of the first pilot symbol 23. The same is true forthe second pilot symbol 48 and the first pilot symbol 25 and the secondpilot symbol 51 and the first pilot symbol 27. Thus, pairs of adjacentsecond pilot symbols, as e.g. the second pilot symbols 42 and 43 as wellas the second pilot symbols 42 and 46 are orthogonal to thecorresponding pairs of the first pilot symbols, e.g. first pilot symbol20 and 21 or first pilot symbol 20 and 23. Thus, orthogonality in thefrequency as well as in the time dimension is ensured.

The same is essentially true for the pilot symbol pattern shown in FIG.4B, which corresponds to the pilot symbol pattern shown in FIG. 3B.Similarly, the pilot symbols of the pilot symbol pattern shown in FIG.4B are alternatingly identical and orthogonal (inverse complex) to thecorresponding first pilot symbols shown in FIG. 3B.

The pilot symbol scheme proposed by the present invention can be appliedto any linear channel estimation algorithm in wireless OFDMcommunications. For the sake of clarity, a simple two pilot symbolaverage based channel estimation algorithm for the pilot symbol patternsof FIG. 3A and FIG. 4A is used as an example in the following furtherdetailed description.

Assuming that the complex values of all first pilot symbols 20, 21, . .. , 28 and the corresponding second pilot symbols having the identicalvalue, i.e. second pilot symbols 42, 44, 47, 50, 52, . . . , is A. Thecomplex value of the second pilot symbols 43, 46, 48, 51, . . . , havinga corresponding orthogonal value is then −A. For all the data symbolsbetween the succeeding pilot symbols 20 and 21 or 42 and 43, respectivechannel estimation values for the first (non-diversity) antenna 5 andthe second (diversity) antenna 6 should be obtained reliably so that theSTTD scheme can be applied.

As stated above, the antenna 11 and the receiving means 12 of the mobileterminal 10 receive the first and the second pilot symbols assuperimposed or combined pilot symbols. Thus, let y₁ and y₂ be thereceived values from the first 20, 21 and the second 42, 43 pilotsymbols. Since the time delay between the first and the second antenna5, 6 is negligible, the following equations are valid:y ₁ =A×h ₁ ¹ A×h ₁ ² +n ₁andy ₂ =A×h ₂ ¹ −A×h ₂ ² +n ₂,

whereby h₁ ¹ is the channel transfer function from the first antenna 5to the receiving antenna 11 for the first pilot symbol 20 with value“A”, h₁ ² is the channel transfer function from the second antenna 6 tothe receiving antenna 11 for the corresponding second pilot symbol 42with value “A”, h₂ ¹ is the channel transfer function from the firstantenna 5 to the receiving antenna 11 for the first pilot symbol 21 withvalue “A”, and h₂ ² is the channel transfer function from the secondantenna 6 to the receiving antenna 11 for the corresponding second pilotsymbol 43 with value “−A”. n₁ and n₂ are the noise values. If y₁+y₂ isused as the channel estimation for the first (non-diversity) antenna 5and y₁−y₂ is used as the channel estimation for the second (diversity)antenna 6, the signals from the first and the second antenna can bedifferentiated, the cross-interference can be eliminated and a reliablechannel estimation for both antennas 5 and 6 can be obtained in theprocessing means 14 of the mobile terminal 10, if the channel transferfunction is assumed to be kept fixed within the interval between thepreceding and the succeeding pilot symbols across the time dimension,i.e. h₁ ¹=h₂ ¹ and h₁ ²=h₂ ².

Thus, in the mobile terminal 10 the signals from the first and thesecond transmitting antenna 5, 6 can be differentiated and consequentlya separate channel estimation for each antenna 5, 6 can be achieved.Since the pilot patterns of the first and the second pilot symbols areorthogonal, the cross-interference from the first and the second antenna5 and 6, can be eliminated. Thus, a STTD scheme can be used in a highdata rate OFDM wireless communication system. It is to be noted, thatthe idea of the present invention can also be applied to OFDM basedbroadband radio access networks (BRAN), like HIPERLAN Type 2 systems. Inthis case, the pilot symbols are transmitted in preamble parts of arespective data burst comprising a preamble part and a data part. Thepilot symbols comprised in the respective preambles should bealternatively identical and orthogonal for the two transmittingantennas.

The invention claimed is:
 1. An apparatus comprising: a transmitteradapted to transmit data symbols and pilot symbols in an OFDMtransmission system, the transmitter configured to: generate said datasymbols and said pilot symbols; and transmit said data symbols and pilotsymbols using a plurality of subcarriers of said OFDM transmissionsystem, wherein said generating said pilot symbols includes generating afirst type pilot symbol and a second type pilot symbol being orthogonalto said first type pilot symbol so that a pilot symbol pattern in afrequency dimension comprises at least said first type pilot symbol tobe transmitted using a first subcarrier and second type pilot symbol tobe transmitted using a second subcarrier, and said pilot symbol patternhas a different pattern in a time dimension from a subsequent pilotsymbol pattern.
 2. The apparatus of claim 1, wherein first pilot symbolsare comprised in said pilot symbol pattern and second pilot symbols arecomprised in said subsequent pilot symbol pattern, at least some of thefirst and second pilot symbols having a same time allocation.
 3. Theapparatus of claim 2, wherein corresponding first and second pilotsymbols have a same frequency and time allocation.
 4. The apparatus ofclaim 2, wherein the first and the second pilot symbols have a regulardistribution in the time and the frequency dimension.
 5. The apparatusof claim 2, wherein the second pilot symbols alternately have identicaland inverse complex value of the corresponding first pilot symbol in thetime and frequency dimension.
 6. The apparatus of claim 1, wherein firstpilot symbols are comprised in said pilot symbol pattern and secondpilot symbols are comprised in said subsequent pilot symbol pattern, atleast some of the first and second pilot symbols having a same timeallocation and being identical and orthogonal to each other.
 7. Theapparatus of claim 1, wherein first pilot symbols are comprised in saidpilot symbol pattern and second pilot symbol s are comprised in saidsubsequent pilot symbol pattern, at least some of the first and secondpilot symbols having a same time allocation and being alternatelyidentical and orthogonal to each other.
 8. The apparatus of claim 7,wherein corresponding first and second pilot symbols having the samefrequency and time allocation are alternatingly identical and orthogonalto each other in the frequency and time dimension.
 9. A transmittingdevice for transmitting data symbols and pilot symbols in an OFDMtransmission system, the transmitting device comprising: circuitryconfigured to generate said data symbols and said pilot symbols; andtransmit said data symbols and pilot symbols using a plurality ofsubcarriers of said OFDM transmission system, wherein said generatingsaid pilot symbols includes generating a first type pilot symbol and asecond type pilot symbol being orthogonal to said first type pilotsymbol so that a pilot symbol pattern in a frequency dimension comprisesat least said first type pilot symbol to be transmitted using a firstsubcarrier and second type pilot symbol to be transmitted using a secondsubcarrier, and said pilot symbol pattern has a different pattern in atime dimension from a subsequent pilot symbol pattern.
 10. Thetransmitting device of claim 9, wherein first pilot symbols arecomprised in said pilot symbol pattern and second pilot symbols arecomprised in said subsequent pilot symbol pattern, at least some of thefirst and second pilot symbols having a same time allocation.
 11. Thetransmitting device of claim 10, wherein corresponding first and secondpilot symbols have a same frequency and time allocation.
 12. Thetransmitting device of claim 10, wherein the first and the second pilotsymbols have a regular distribution in the time and the frequencydimension.
 13. The transmitting device of claim 10, wherein the secondpilot symbols alternately have identical and inverse complex value ofthe corresponding first pilot symbol in the time and frequencydimension.
 14. The transmitting device of claim 9, wherein first pilotsymbols are comprised in said pilot symbol pattern and second pilotsymbols are comprised in said subsequent pilot symbol pattern, at leastsome of the first and second pilot symbols having a same time allocationand being identical and orthogonal to each other.
 15. The transmittingdevice of claim 9, wherein first pilot symbols are comprised in saidpilot symbol pattern and second pilot symbols are comprised in saidsubsequent pilot symbol pattern, at least some of the first and secondpilot symbols having a same time allocation and being alternatelyidentical and orthogonal to each other.
 16. The transmitting device ofclaim 15, wherein corresponding first and second pilot symbols havingthe same frequency and time allocation are alternatingly identical andorthogonal to each other in the frequency and time dimension.
 17. Amethod for transmitting data symbols and pilot symbols in an OFDMtransmission system, the method comprising: generating said data symbolsand said pilot symbols; and transmitting said data symbols and pilotsymbols using a plurality of subcarriers of said OFDM transmissionsystem, wherein said generating said pilot symbols includes generating afirst type pilot symbol and a second type pilot symbol being orthogonalto said first type pilot symbol so that a pilot symbol pattern in afrequency dimension comprises at least said first type pilot symbol tobe transmitted using a first subcarrier and second type pilot symbol tobe transmitted using a second subcarrier, and said pilot symbol patternhas a different pattern in a time dimension from a subsequent pilotsymbol pattern.
 18. The transmitting method of claim 17, wherein firstpilot symbols are comprised in said pilot symbol pattern and secondpilot symbols are comprised in said subsequent pilot symbol pattern, atleast some of the first and second pilot symbols having a same timeallocation.
 19. The transmitting method of claim 17, wherein first pilotsymbols are comprised in said pilot symbol pattern and second pilotsymbols are comprised in said subsequent pilot symbol pattern, at leastsome of the first and second pilot symbols having a same time allocationand being identical and orthogonal to each other.
 20. The transmittingmethod of claim 17, wherein first pilot symbols are comprised in saidpilot symbol pattern and second pilot symbols are comprised in saidsubsequent pilot symbol pattern, at least some of the first and secondpilot symbols having a same time allocation and being alternatelyidentical and orthogonal to each other.